Acute Respiratory Distress Syndrome is defined by the Berlin Definition:

  • Acute onset (< 7days from insult)
  • Bilateral infiltrate on CXR / CT
  • Hypoxia: P/F Ratio < 300 on a minimum of 5 cm H2O PEEP
  • Not fully explained by cardiac failure or volume overload

ARDS is a heterogeneous syndrome with multiple causes. It is a diffuse, inflammatory process within the lungs resulting in increased vascular permeability. This results in hypoxia, poor compliance, dead space and shunt.

The P/F Ratio

The P/F (or PaO2/FiO2) ratio is a marker of ARDS severity. It is the measured PaO2 for a given FiO2 (or inspired oxygen fraction). For example if the PaO2 is 150 on an FiO2 of 0.5, the P/F ratio is 150/0.5 = 300.

The ratio can be used to classify ARDS severity, which correlates with increasing mortality:

  • Mild < 300
  • Moderate 100 – 300
  • Severe < 100


Lung Protective Ventilation

This consists of a set of parameters designed to help minimise the risk of ventilator induced lung injury.

In ARDS the lungs have poor compliance, so high pressures are needed for a given change in volume.

  • Tidal Volumes: 4 – 6mls/kg Predicted Body Weight
  • P plateau < 30 cm H2O
    • Driving Pressure (P plateau – PEEP) < 15 cm H2O
    • High driving pressures have been shown to increase mortality
  • A high PEEP strategy may be needed in patients with moderate – severe ARDS
  • Tolerance of a mild respiratory acidosis may be needed (“Permissive Hypercapnia”) – achieving an adequate minute ventilation may require injurious pressures or volumes.


Proning is the technique to position a patient on their front. This has been shown to reduce mortality in patients with a P/F ratio < 150. Patients remain prone for 12 – 16 hours at a time.

Other management strategies that can help:

  • The following have a weaker evidence base than the two strategies above:
    • Paralysis: in moderate to severe ARDS the use of a continuous neuromuscular blockade infusion has been shown to improve oxygenation and increase ventilator free days. Often a cistracurium infusion is used if required.
    • A conservative IV fluid regime
    • ECMO

Strategies that do not show any benefit:

  • Recruitment Manoeuvres
  • High Frequency Oscillatory Ventilation

References and Further Reading


Prone Position in acute respiratory distress syndrome

Formal guidelines: management of acute respiratory distress syndrome

FICM / ICS: Guidance for prone positioning in intensive care

Author: George Walker